Within the United States, there is considerable interest in developing alternative energy sources to reduce dependence on foreign oil and nonrenewable energy. The use of ethanol as a fuel has become increasingly prevalent in recent years.
One way to meet the demand for ethanol production is to convert sugars found in biomass, i.e., materials such as agricultural wastes, corn hulls, corncobs, cellulosic materials, and the like to produce ethanol. In biomass conversion, microorganisms are used as biocatalysts to convert cellulosic materials to usable end products, such as ethanol. Efficient biomass conversion in large-scale industrial applications requires a microorganism that is able to tolerate high concentrations of sugar and ethanol, and which is able to ferment more than one sugar simultaneously.
Biomass commonly contains xylose at relatively high concentrations, i.e., as high as 25% of the total dry weight. The D-xylose content of hardwood species and herbaceous angiosperms is about 17% and 31% of the total dry weight, respectively. Because agricultural residues, pulping wastes, and fast-growing hardwood species have a high xylose content, the potential economic and ecologic benefits of converting xylose in these renewable materials are significant. In order for biomass conversion to be economically feasible, a practical, large-scale use must be found for xylose.
The pentoses D-xylose and L-arabinose are among the most difficult sugars in biomass to metabolize. Bacteria can ferment pentoses to ethanol and other co-products, and bacteria with improved ethanol production from pentose sugars have been genetically engineered. However, these bacteria are sensitive to low pH and high concentrations of ethanol, their use in fermentations is associated with co-product formation, and the level of ethanol produced remains too low to make using these bacteria in large-scale ethanol production economically feasible.
In general, industrial producers of ethanol strongly favor using yeast as biocatalysts, because yeast fermentations are relatively resistant to contamination, are relatively insensitive to low pH and ethanol, and are easier to handle in large-scale processing. Many different yeast species use xylose respiratively, but only a few species use xylose fermentatively. Fermentation of xylose to ethanol by wild type xylose-fermenting yeast species occurs slowly and results in low yields, relative to fermentation rates and ethanol yields that are obtained with conventional yeasts in glucose fermentations. In order to improve the cost effectiveness of xylose fermentation, it is necessary to increase the rate of fermentation and the ethanol yields obtained.
What is needed in the art are new yeast strains and methods for fermenting xylose to produce ethanol.